Cuschieri, Joseph M.

In this dissertation, the digital signal processing techniques required for a 3-D sonar imaging system are examined. The achievable performance of the generated images is investigated by using a combination of theoretical analysis, computer simulation and field experiments. The system consists of a forward looking sonar, with separate projector and receiver. The projector is a line source with an 80 degrees by 1.2 degree beam pattern, which is electronically scanned within a 150 degree sector. The receiver is a multi element line array, where each transducer element has a directivity pattern that covers the full sector of view, that is 150 degrees by 80 degrees. The purpose of this sonar system is to produce three dimensional (3-D) images which display the underwater topography within the sector of view up to a range of 200 meters. The principle of operation of the proposed 3-D imaging system differs from other commonly used systems in that it is not based on the intensity of backscatter. The geometries of the targets are obtained from the delay and direction information that can be extracted from the signal backscatter. The acquired data is further processed using an approach based on sequential Fourier transforms to build the 3-D images. With careful selection of the system parameters, the generated images have sufficient quality to be used for AUV tasks such as obstacle avoidance, navigation and object classification. An approach based on a sophisticated two dimensional (2-D) autoregressive (AR) model is explored to further improve the resolution and generate images with higher quality. The real time processing requirements for image generation are evaluated, with the use of dedicated Digital Signal Processing (DSP) chips. A pipeline processing model is analyzed and developed on a selected system.

The response and the dynamic stability of thin cylindrical shells excited by a point force with an internal heavy medium (water) moving with a constant mean flow velocity are investigated. Two sets of analysis are discussed, one for infinitely long shells and one for shells of finite length. The infinite condition applies to shells which are sufficiently long to be considered infinite. In this case, the solution is obtained by means of a spatial Fourier transform in the axial direction and a modal decomposition in the circumferential direction. Using this solution, input and transfer accelerances are determined. The results of this part of this analysis show that input accelerances are globally preserved as the mean flow velocity changes. For the transfer accelerances, broadband peaks appear which are caused by the phase matching between propagating waves of different mode numbers. These broadband peaks are shifted and modified by the mean flow velocity. For shells of finite length, simply supported boundary conditions are assumed. In this case, the response is obtained by using a normal mode expansion of the in vacuo shell and the Kirchhof-Helmholtz equation derived for a fluid moving at constant flow velocity and bounded by a perfectly rigid cylinder. For finite shells, the main effect of the flow on the response is to decrease the natural frequencies of the shell. The extent of the change in frequency depends on the circumferential and axial mode numbers. Experimental results are also presented for a pipe shell of radius 0.025m and wall thickness 1.5mm. These results are compared with the analytical results for similar shell and flow condition and the agreement is very good. Using the analysis developed for the response, results are presented on the instabilities that can be induced by the flow. It is found that these instabilities are not restricted to finite pipes, but can also exist for cylindrical shells of infinite extent.

The total vibrational power flow in connected plate structures is investigated using an analytical "Power Flow" approach. The effects of shear and rotary inertia on the flexural wave transmission and the influence of in-plane wave generation at structural discontinuities are included in the analytical model. In formulating a Power Flow model, the structure is divided into substructures whose responses may be determined analytically to obtain expressions for the input and transfer mobilities of the substructures. For the case of plate-type structures joined along a line, the mobilities are functions of both frequency and space. The power transmission between the individual plate substructures is then written as a function of these mobility expressions. The structure of concern in this dissertation consists of two plates connected in an L-configuration. In obtaining the expressions for the mobilities, the vibrational response of the individual plates is determined by solving the appropriate equations of motion. In this study the antisymmetric (flexural) motion is described using Mindlin's (1951) thick plate approximation to the three-dimensional equations of motion. The applicability of this thick plate formulation is limited to frequencies below the frequency of the first antisymmetric mode of thickness-shear vibration of the plate. The symmetric (in-plane) motion of the plates is described using the generalized theory of plane stress which neglects the direct coupling of the in-plane motion with the thickness vibration modes, and is therefore valid only for frequencies which are lower than the frequency of the first mode of pure thickness vibration of the plate. The results for the power transmission in the L-plate obtained using the Power Flow formulation are verified at high frequencies by comparison with the results obtained using the Statistical Energy Analysis (SEA) technique. The SEA formulation for the L-plate is based on Mindlin's equations for flexural motion and the theory of generalized plane stress for in-plane vibration. The results of the Power Flow formulation are verified at low frequencies by the results obtained using a Finite Element model of the L-shaped plate.